Determining Triplets Zygosity

Have you ever wondered if triplets can be identical
like twins too? Triplets are three babies carried in the womb during a single
pregnancy. Like twins, triplets and other higher order multiples (quadruplets,
quintuplets etc) they can be categorised by their zygosity this is the type
of triplets they are, so whether they are identical or fraternal (non-identical).

Most
triplets are trizygotic, meaning that each child forms from a separate
zygote, or egg/sperm combination. They are commonly described as
"fraternal" multiples and share the same genetic similarities as any
siblings. However, it is not uncommon for triplets to be dizygotic, which
occurs when two eggs are fertilized by sperm and one of the fertilized eggs
splits into two. This creates a set of identical twins and a third 'single' triplet (sometimes referred to as a pair and a spare!). Essentially, two of the triplets are monozygotic, or identical twins, sharing the same general DNA characteristics, while the
third multiple has a unique genetic heritage.

Monozygotic
Triplets

It is
rare for triplets to be completely monozygotic, meaning that all three children
formed from a single egg that split three ways, or split into two and then one
of the two split again, ultimately resulting in three embryos with the same
general DNA characteristics. Some monozygotic triplets are actually quadruplets
where one embryo has vanished or been reabsorbed.

Monozygotic triplets are always
of the same gender, either all boys or all girls.

Recently,
identical triplets made headlines when a monozygotic triplet pregnancy resulted
after a single embryo was implanted in an in-vitro procedure. Allison Penn gave
birth to Logan, Eli and Collin in March 2008. Her doctors identified
the situation as the only known case of a single embryo transfer resulting in
triplets.

What are
the Odds?

Research
varies on the instances of monozygotic triplets. Estimates range between one in
60,000 and one in 200 million. When a British woman gave birth to
identical girls in Austria in August 2007 without fertility interventions, the
event was termed as something that only happens "in about one in 200,000
pregnancies", or in one out of every six triplet pregnancies. The article
estimated that about ten sets of identical triplets are born in Britain each
year. In 2012, there were 185 sets of triplets born in the UK and this has
remained steady between 150-180 sets for the last few years. Triplet births
have significantly dropped over the years due to regulations on the number of
embryos that can be transplanted with the majority of IVF treatments now either
using two embryos or single embryo transplant (SET).

In 2013,
a set of identical girls were born in California. Dr. William Gilbert, director
of Women's Services for Sutter Health in Sacarmento, explained that the
occurrence is so rare that it was difficult to accurately measure
statistically. He placed the odds at one in 1 million to one in 100 million.

There doesn't seem to be a clear
definition of what exactly the odds are, perhaps because it is rare. Another
explanation of the variance may be in defining triplet conceptions /pregnancies
and live triplet births. A monozygotic triplet pregnancy carries the risks of
a monozygotic pregnancy with a shared placenta, and the additional risks of a triplet
pregnancy. A pregnancy of this type is more likely to encounter problems such
as pre-eclampsia, preterm labour, and even Twin to Twin Transfusion
Syndrome (TTTS).